1. Field of the Invention
The present invention relates to a plasma processing apparatus used in semiconductor process plasma treatments such as ashing and etching, a method for operating the plasma processing apparatus, and a plasma processing method using the plasma processing apparatus.
The present invention also further relates to a system for designing a matching circuit that matches the load impedance of a plasma processing chamber of the plasma processing apparatus.
2. Description of the Related Art
FIG. 15 shows a typical conventional single-frequency excitation plasma processing apparatus which is used in plasma processes such as a chemical vapor deposition (CVD) process, a sputtering process, a dry etching process, and an ashing process. This plasma processing apparatus includes a matching circuit 2C between a radio frequency generator 1 and a plasma excitation electrode 4. The matching circuit 2C matches the impedance between the radio frequency generator 1 and a plasma processing chamber CN.
The RF power from the radio frequency generator 1 is fed into the plasma excitation electrode 4 through the matching circuit 2C. The matching circuit 2C is accommodated in a conductive matching box 2. The plasma excitation electrode 4 and a feed plate 3 are covered with a conductive chassis 21.
An annular projection 4a is provided on the bottom face of the plasma excitation electrode (cathode) 4, and a shower plate 5 having many holes 7 comes into contact with the projection 4a below the plasma excitation electrode 4. The plasma excitation electrode 4 and the shower plate 5 define a space 6. A conductive gas-feeding pipe 17 is connected to the space 6 and is provided with an insulator 17a at the middle portion thereof to insulate the plasma excitation electrode 4 and the gas source.
Gas from the gas-feeding pipe 17 is introduced into a plasma processing space 60 surrounded by a chamber wall 10, via the holes 7 in the shower plate 5. An insulator 9 is disposed between the chamber wall 10 and the plasma excitation electrode (cathode) 4 for insulation therebetween. An exhaust system is omitted from the drawing.
A wafer susceptor (susceptor electrode) 8, which holds a substrate 16 and also functions as another plasma excitation electrode, is supported by a shaft 13 in the plasma processing space 60.
The lower portion of the shaft 13 and the chamber bottom 10A are hermetically sealed with bellows 11. The wafer susceptor 8 and the shaft 13 can be moved vertically by the bellows 11 so as to control the distance between plasma excitation electrodes 4 and the susceptor electrode 8. The wafer susceptor 8 is DC-grounded and has the same DC potential as that of the chamber wall 10.
In this plasma processing apparatus, RF power of a frequency of about 40.68 MHz is generally fed to generate a plasma between the two electrodes 4 and 8 for plasma treatments such as CVD, sputtering, dry etching, and ashing.
In general, the main components, including the plasma processing chamber CN, of the plasma processing apparatus are manufactured by an apparatus manufacturer whereas the matching circuit 2C and the radio frequency generator are manufactured by an electrical manufacturer. Hence, a user of the plasma processing apparatus must perform impedance matching between the plasma processing chamber CN and the radio frequency generator 1 by adjusting the matching circuit 2C for each plasma treatment such as sputtering, dry etching, or ashing. Herein the impedance (load impedance) of the plasma processing chamber CN before the plasma generation is designated as Z0 or after the plasma generation as Z1.
The impedance Z0 is partially determined in a designing process by the manufacturer and can be precisely measured for that specific chamber; however, manufactured plasma processing chambers have different impedances due to dimensional differences generated in the manufacturing processes.
Furthermore, after the plasma is generated, the impedance Z1 varies with process parameters including the flow rate of gas used, the degree of vacuum in the plasma processing chamber, and the distance between the two electrodes 4 and 8. Thus, the impedance Z1 will differ from one plasma treatment to the next in the same plasma processing apparatus. Accordingly, the impedance Z1 is a parameter that should be determined after optimization of the plasma treatment that is performed by the user.
For example, in a dry etching apparatus, the impedance Z1 varies with the type of material of thin film material to be etched and etching conditions such as an etching rate and the shape of a portion to be etched. Also in a film deposition apparatus, the impedance Z1 varies with the process gas composition for forming a thin film and deposition conditions such as a deposition rate and the structure of the thin film. Thus, the actual impedance Z1 of their apparatuses cannot be provided as design information by the apparatus manufacturer and the RF generator manufacturer on a delivery of the apparatuses to users.
Accordingly, the RF generator manufacturer sets a wide margin for user adjustment of the impedance of the matching circuit 2C so that the matching circuit 2C can be used for plasma processing chambers having different impedances Z0 and processes having different impedances Z1. The users must adjust the output impedance of the matching circuit 2C to the impedance Z0 before plasma treatment and then to the impedance Z1 after plasma discharge so as to stabilize the plasma discharge.
At the beginning of the plasma treatment, namely, at or after the beginning of the plasma discharge, the output impedance of the matching circuit 2C is adjusted to the impedance Z1 by resetting a tuning capacitor and a load capacitor to the maximum or minimum. Then, the capacitances of the tuning capacitor and the load capacitor of the matching circuit 2C are adjusted by a control circuit 14 so that the output impedance of the matching circuit 2C is adjusted to the impedance Z1 of the plasma chamber after plasma discharge.
The control circuit 14 adjusts the capacitances of the load capacitor and the tuning capacitor to minimize the electric power such as spurious power of the reflected waves as measured by a reflected wave detector 15. The reflected wave detector 15 is disposed between the matching box 2 and the radio frequency generator 1. The load capacitor and the tuning capacitor are variable capacitors of which capacitances can be varied, for example, by rotation of motors (not shown in the drawing) that are driven by the control circuit 14.
In the above conventional plasma processing apparatus, the adjustment of the output impedance of the matching circuit 2C to the impedance Z0 requires many hours of work before plasma treatment commences. Furthermore, a user of the plasma processing apparatus cannot know the exact impedance Z0; hence, the control circuit 14 cannot adjust the output impedance of the matching circuit 2C to the impedance Z0 in some cases. In such cases, plasma is not discharged because of impedance mismatching.
Furthermore, the impedance Z1 of the matching circuit 2C after plasma discharge is not stored in the conventional plasma processing apparatus; thus, the adjustment of the output impedance to the impedance Z1 also requires many hours of work.
When plasma discharge starts in the plasma processing chamber CN, the output impedance of the plasma processing chamber CN varies from Z0 to Z1. Since the user does not know the value of the impedance Z1, the output impedance cannot be exactly set to Z1. Thus, the effect of the plasma discharge decreases due to mismatching.
Further, in the conventional configuration, the inductance of a tuning inductor of the conventional plasma processing apparatus must be large in order to achieve a wide margin of the output impedance of the matching circuit 2C. Unfortunately, this results in a large parasitic RF resistance component for the tuning inductor of the plasma processing apparatus. As a result, power loss in the matching circuit 2C increases.
For example, for a radio frequency generator 1 having a frequency of 40.68 MHz, the impedance Z1 of the plasma processing chamber is 3.6 Ω+j1.4 Ω. Referring to FIG. 16, the matching circuit includes a resistor 101 and an inductor 102. The resistor 101 represents a parasitic RF resistance of a feed line between a coaxial cable 1A from the radio frequency generator 1 and the tuning inductor and has a resistance of 0.33 Ω at 40.68 MHz. The inductor 102 is a parasitic inductance of the feed line and has an inductance of 161 nH.
The relationship between constants (design circuit constants) of individual elements constituting the matching circuit will now be described with reference to the Smith chart shown in FIG. 17. The Smith chart is normalized by the characteristic impedance 50 Ω of the power supply system.
The point A represents the characteristic impedance (50 Ω) of the power supply system including the radio frequency generator 1 and the coaxial cable 1A at the input site of the matching circuit 2C. The point B represents the shift of the impedance caused by the resistor 101, and the point C represents the shift of the impedance caused by the inductor 102. Similarly, the point D represents the shift of the impedance caused by a load capacitor 106, the point E represents the shift of the impedance by a resistor 103, and the point F represents the shift of the impedance by an inductor 104.
In the Smith chart shown in FIG. 17, the point G represents the final output impedance of the matching circuit 2C. Since this final output impedance has a conjugate complex value with respect to the load impedance of the plasma processing chamber CN, the impedance Z1* at the point G is 3.6 Ω−j1.4 Ω, wherein the impedance Z1* is the conjugate complex impedance of the impedance Z1. Thus, the final impedance matching from the point A to the point G is achieved by the shift from the point F to point G by the tuning capacitor 105.
Since the actual impedance Z1 depends on the type of the plasma treatment and operating conditions, all of which are unknown at the time the circuit is designed, the inductor 104 is provided with a large adjustable range so that the adjustable range of the tuning capacitor 105 (i.e., the distance between the point E and the point F) becomes correspondingly larger. As a result, the parasitic resistor 103 of the inductor 104 has a large resistance of 2.72 Ω, resulting in inefficient power consumption. A large resistance of the parasitic resistor 103 causes a shift of the impedance at the connection P in FIG. 16 from the point E on arc E-G to the point D.
A change in the impedance from the point C to the point D requires that the load capacitor 106 have a large capacitance value. Since a large current flows through the load capacitor 106, a large current loss occurs in the matching circuit (an increase in fed current loss). Accordingly, a current flowing in the resistor 101 also increases in the conventional matching circuit for the same reasons, and thus much power is consumed in the resistor 101. Consequently, a large amount of electric power is consumed in the resistor 101 and the parasitic resistor 103 of the matching circuit.
A variable inductor having a movable terminal is used in place of the inductor 104 for solving the above problem. If the variable inductor has high contact resistance at the connection of the terminal, large heat is generated at the connection due to large electrical loss. Such heat generation would cause damage of the inductor.
Furthermore, the variable range of the inductor is not optimized nor fixed. Thus, the inductance, which typically varies during maintenance, must be readjusted. In addition, the shift of the terminal on the inductor coil inhibits fine adjustment of the impedance between different plasma treatments. Moreover, the impedance Z0 and impedance Z1 of the plasma processing chamber are unknown after the adjustment, as in the conventional plasma treatment apparatus.